This is a request for extension of Merit Award DA015835. This Award focused on a fundamental question: how do psychomotorstimulants modulate synaptic plasticity at excitatory synapses? To address this, we conducted the first studies of AMPA receptor trafficking, a critical mechanism for regulating synaptic strength, in primary cultures from the nucleus accumbens (NAc) and other reward-related brain regions. We showed that D1 receptors facilitate AMPA receptor synaptic incorporation, providing a mechanism to explain D1 receptor-dependent facilitation of LTP and learning in the normal brain. Abnormal engagement of this mechanism during unregulated DA release may account for maladaptive plasticity during repeated exposure to cocaine. In the next funding period, we will continue to address fundamental questions about plasticity in reward-related brain regions. In particular, we want to understand mechanisms underlying our recent observation that cell surface AMPA receptor levels in the NAc are increased in 2 animal models of addiction: behavioral sensitization and incubation of cocaine craving. Aim 1will determine the subunit composition of AMPA receptor populations in NAc and other addiction-related brain regions. It is critical to know subunit composition, as this determines many features of plasticity. Aim 2 will focus on TARPs (transmembrane AMPA receptor regulatory proteins), a recently discovered family of proteins that regulates AMPA receptor trafficking and function. We will examine TARP localization and function in NAc neurons, the role of TARP phosphorylation in AMPA receptor trafficking, and the role of TARPs in AMPA receptor adaptations that occur in behavioral sensitization and incubation. Aim 3 will focus on synaptic scaling, an important form of homeostatic plasticity in which surface and synaptic expression of AMPA receptors upregulates in response to prolonged hypoactivity of excitatory transmission and downregulates in response to prolonged hyper- activity. We will characterize changes in expression and localization of AMPA receptor subunits and TARPs during synaptic scaling in NAc neurons, as well as TARP phosphorylation. We hypothesize that synaptic scaling, triggered by hypoactivity of cortical inputs during cocaine withdrawal, is responsible for AMPA receptor upregulation in the NAc during sensitization and incubation. Public health relevance: Understanding plasticity mechanisms in addiction will help develop therapies aimed at unlearning cues that elicit craving.